With recent advances in electronic technology, portable electronic devices, such as cellular phones, portable personal computers, personal data assistances (PDAs) and portable game consoles, have been spread rapidly. Accordingly, there has been an increasing demand for electricity storage devices, such as secondary batteries, as power supplies for portable electronic devices. Among them, lithium ion secondary batteries are widely used as power supplies for portable electronic devices due to their high electromotive force and high energy density and relatively high adaptability to miniaturization.
In order to increase the versatility of portable electronic devices, they are required to provide higher performance. For example, they are required to be more lightweight, more compact, and more multifunctional. Batteries used as power supplies for such portable electronic devices are required to have higher energy densities, for example. An effective approach to increase the energy density of a battery is to use an electrode active material having a high energy density. Therefore, active research and development of novel materials having higher energy densities have been conducted for both positive and negative electrode active materials.
For example, the use of an organic compound capable of reversible redox reactions as an electrode active material has been studied. Organic compounds have specific gravities of about 1 g/cm3 and are lighter in weight than inorganic oxides, such as lithium cobalt oxide, which have been conventionally used as electrode active materials. Therefore, the use of an organic compound as an electrode active material makes it possible to obtain an electricity storage device having a high weight energy density. The use of an organic compound free of heavy metals as an electrode active material also makes it possible to reduce risks of exhaustion of rare metal resources, fluctuations in the prices of such resources, environmental pollution by leakage of heavy metals, etc.
As a specific example of the use of such an organic compound, it has been proposed to use 9,10-phenanthrenequinone as a positive electrode active material and lithium ions as counter ions in a coin-type secondary battery containing a non-aqueous electrolyte solution (see Patent Literature 1). In the battery of Patent Literature 1, the positive electrode contains 9,10-phenanthrenequinone and a conductive agent such as carbon. The counter electrode to the positive electrode is made of metallic lithium. The electrolyte is made of a propylene carbonate solution in which lithium perchlorate is dissolved at a concentration of 1 mol/L.
However, 9,10-phenanthrenequinone is readily soluble in an electrolyte (liquid electrolyte). The solubility of 9,10-phenanthrenequinone greatly depends on the components and amount of the electrolyte and the configuration of the battery. Patent Literature 1 does not describe the dissolution of the positive electrode active material in the electrolyte. However, in view of the fact that the discharge capacity decreases as the number of charge-discharge cycles increases, it is considered that the dissolution of the positive electrode active material into the electrolyte is not sufficiently inhibited. For the practical use of 9,10-phenanthrenequinone as an electrode active material, it is essential to inhibit the dissolution thereof into the electrolyte.
For the purpose of inhibiting the dissolution of a phenanthrenequinone compound into an electrolyte, it has been proposed to use, as an electrode active material, a polymer compound having a phenanthrenequinone skeleton in the main chain (see Patent Literatures 2 and 3). Patent Literature 2 discloses a polymer compound obtained by polymerization of 9,10-phenanthrenequinones at their 2- and 7-positions. Patent Literature 3 discloses a polymer compound in which aromatic compounds such as a phenyl group and a thiophene group as linkers are bonded to the 2- and 7-positions or the 3- and 6 positions of 9,10-phenanthrenequinones. The use of the polymer compounds disclosed in Patent Literatures 2 and 3 as electrode active materials could help inhibit the dissolution of 9,10-phenanthrenequinone into an electrolyte without impairing the electrochemical properties of 9,10-phenanthrenequinone.